Several publications are referenced herein by Arabic numerals within parenthesis. Citations for these references may be found at the end of the written description of the present invention. The disclosures of these publications are hereby incorporated herein by reference in their entirety, unless otherwise noted.
Ceramic materials are currently widely used in advanced engineering applications, including insulating microelectronic substrates and forming structures such as metallurgical crucibles, liquid metal filters, components (e.g., nozzles, pump valves, seals, etc.) for handling highly corrosive liquids, and medical devices, to name a few. Currently, alumina is the most prevalent ceramic material used in advanced engineering applications, including dielectric substrates, biomedical devices, automotive parts, and other structures and components (1-3). Like most ceramics, the mechanical properties of sintered alumina are greatly compromised by microstructural heterogeneities introduced during shape forming by powder compaction, slip casting, injection molding, and other methods. Common heterogeneities are porosity gradients, isolated pores, cracks, and agglomerates (4-5).
Ceramic materials are typically produced using a sequential process of mixing ceramic powder with an organic liquid carrier (e.g., alcohols, ketones, polyethylene wax, and vinyl compounds) to form a moldable slurry, forming the slurry into a desired configuration (e.g., by injection molding or plastic shaping), thermally treating to evaporate or pyrolyze the carrier, and kiln firing. During the process, microstructure heterogeneities can be introduced during any one or more of these steps, for example, synthesizing of the powder, evaporating the carrier, and kiln firing. Such heterogenieties can include porosity gradients, impurities, isolated pores, cracks, agglomerates, and more significantly, nonuniform -green density that can produce nonuniform shrinkage stresses during drying. Such nonuniform green density can lead to cracks and/or shape distortion of the kiln-fired (sintered) ceramic product.
Traditional methods of quality control, including pycnometry, mechanical property measurements, fractography, and mercury porosimetry, are difficult to use when attempting to optimize the many different processing variables that ordinarily affect microstructure-defect evolution in ceramic production (6). The conventional apparatuses and techniques usually involve slow and costly trial-and-error procedures that are destructive and typically require significant alteration of the ceramic piece for analysis. For example, conventional techniques typically require cutting a ceramic piece for analysis into many small pieces and then performing microscopy, density measurements (by Archimedes immersion principle) and the like, on every piece. This is a potentially significant problem in green body analysis, due to the fragile nature of green bodies that often require application of preservation techniques to the green body before analysis. Such preservation techniques include, but are not limited to, chemical fixation or drying, followed by partial sintering, cutting, and polishing. Consequently, there is an increased potential for the introduction of contaminants through the preservation process.
Researchers are currently attempting to use ultrasound and x-rays to map density gradients in ceramics (6-8), but there are problems adapting these methods to production environments. For example, x-ray methods are not always fast enough or affordable in a production setting. A concern with ultrasound is that it requires liquid coupling media, which often disintegrates green bodies. xe2x80x9cAir-coupledxe2x80x9d ultrasound was recently developed to overcome the problem of disintegration, but this technology has not been fully developed for ceramic production (9-10).
Thus, what is yet needed is a method and apparatus for analyzing ceramics. In particular, what is needed is a method and apparatus for analysis of a ceramic, such as for the analysis of the density, density gradients and/or microcracks in a given ceramic sample, which can be accomplished relatively quickly, easily, without destruction of the ceramic sample during evaluation, and preferably at a lower cost than conventional methods for detecting density and/or microcracks.
One aspect of the present invention provides an apparatus for analyzing ceramic density gradients and/or microcracks, including an excitation source capable of exciting a component in a ceramic surface; a charge-couple device; and a processor, operably linked to the charge-couple device, wherein the processor is at least capable of integrating a luminescence intensity generated by exciting the component. Preferably, the excitation source comprises a laser capable of emitting energy having an associated wavelength within an ultraviolet spectrum or an infrared spectrum. More preferably, the excitation source is capable of exciting an impurity ion (e.g., Cr+3) that in turn luminesces.
Another aspect of the present invention provides an apparatus for analyzing microcracks, including an excitation source capable of exciting a component in a ceramic surface; a detector operably linked to the excitation source; a translation portion capable of adjusting a position of a ceramic in at least one direction relative to the excitation source; and processor operably linked to the detector, wherein the processor is at least capable of comparing a measurement obtained from exciting the component to a predetermined background measurement. Preferably, the excitation source comprises a laser capable of emitting energy having an associated wavelength within an ultraviolet spectrum or an infrared spectrum. Preferably, the excitation source is capable of exciting a chromium ion.
A further aspect of the present invention provides a method for analyzing density of a ceramic comprising exciting a component on a surface of the ceramic by exposing the surface to energy. The method further includes the step of obtaining a measurement of an emitted energy from the component. The method additionally includes comparing the measurement of the emitted energy from the component with a predetermined reference measurement so as to obtain a density for said ceramic.
Another aspect of the present invention provides a method for analyzing a density gradient and/or microcracks of a ceramic comprising exciting a component on a surface of the ceramic. The method further includes obtaining a measurement of an emitted energy from the component. Additionally, the method includes imaging the ceramic so as to visually indicate a density gradient and/or microcracks associated with the measurement.
Another aspect of the present invention provides a method for analyzing a density gradient and/or microcracks of a ceramic comprising scanning in at least one direction of a surface of said ceramic with an input energy so as to excite components from the surface of the ceramic. Additionally, the method includes obtaining a measurement of an emitted energy from the components and observing the measurement of the emitted energy so as to indicate density gradients and/or microcracks associated with the measurement.
Yet another aspect of the present invention provides a method for analyzing a density gradient and/or microcracks of a ceramic comprising chemically reacting a ceramic with a chemically reactive solution so as to cause emission of energy from components in the ceramic. Additionally, the method includes observing the emitted energy so as to indicate density gradients and/or microcracks associated with the emitted energy.
In the methods described above, the ceramic can be a sintered ceramic or a green ceramic. In a preferred embodiment, the ceramic comprises alumina, wherein the component comprises a chromium ion.
In another aspect of the invention there is provided a chemiluminescent kit comprising one or more chemiluminescent materials which synergistically impart enhanced luminescence to ceramics.